Oxide particles are materials used in a wide range of fields such as catalysts, conductive materials, magnetic materials, secondary electron emission materials, luminous bodies, heat absorbers, energy storage bodies, electrode materials, color materials, and the like. As characteristics of the oxide particles change depending on the particle size, oxide particles having different particle diameters or crystallinity are required depending on the purpose and requirements. Particularly, characteristics of the particles greatly different from those in the bulk state may be expressed by micronization, and oxide particles are widely required materials in the future.
For example, magnetite is a kind of iron oxide represented by chemical formula Fe3O4 (Fe(II)Fe(III)2O4), and is a material which has been widely used from long ago. In particular, magnetite particles are chemically stable particles having relatively large magnetic properties and have been widely used for magnetic recording media, magnetic fluids or the like in the information recording field, or for applications as magnetic toner, carrier, pigment or the like in the image recording field. In addition, in recent years, magnetite particles have been also used for contrast media in NMR, thermotherapy of cancer, or the like in the medical field, and practical use in various fields is expected.
Similarly, cerium oxide is a kind of oxide represented by chemical formula CeO2 (IV) and has been widely used as an abrasive for a long time. In recent years, as a method for producing nanosize cerium oxide particles has been developed, utilization thereof in new applications such as ultraviolet absorbing agents, solid electrolytes, catalyst carriers or the like in addition to abrasives is progressing.
Methods of producing oxide particles known today include sol-gel method, co-precipitation method, hydrothermal synthesis and the like.
For example, as a method of producing magnetite, a method of co-precipitating ferrous ions (Fe2+) and ferric ions (Fe3+) in an alkaline solution, a method of oxidizing a ferrous hydroxide solution with air, a method of reducing iron oxide (α-Fe2O3) or iron hydroxide (α-FeOOH) under a hydrogen atmosphere and the like have been known from long ago. However, there are problems that the obtained magnetite particles tend to become coarse, and it is difficult to obtain nanometer order magnetite having a primary particle size of 100 nm or less, and dry heat treatment or high temperature are required.
As characteristics of nanoparticles, particles generally tend to aggregate due to the influence of surface energy as the particle diameter decreases. In addition, since the magnetite particles have magnetic aggregability, improvement of dispersibility is a particularly serious problem. Single crystal magnetite particles having the most stable surface state in their average particle size are desirable for improving the dispersibility, and improvement of durability against temperature, light or solvent and magnetic characteristics and the like are expected. Thus, methods of producing single crystal magnetite particles have been proposed for a long time as follows.
Patent Literature 1 discloses a method of forming magnetite particles comprising adding an oxidizing agent to an alkaline aqueous solution subjected to deoxidation treatment, and adding a soluble amount of a divalent iron ion to the alkaline aqueous solution to which the oxidizing agent is added, and stirring the alkali aqueous solution to which the divalent iron ion is added.
Patent Literature 2 discloses a method of neutralizing the ferric salt aqueous solution with an alkali aqueous solution at a temperature in the range of 5 to 40° C., filtering the formed ferric hydroxide, washing the ferric hydroxide with water, dispersing the ferric hydroxide in water, and adding to the dispersion a reducing agent in an amount sufficient to reduce one third of the total ferric ions to form a slurry having a pH in the range of 7 to 11, and then subjecting the slurry to hydrothermal reaction at a temperature of 120 to 200° C.
In the method as described in Patent Literature 1 or Patent Literature 2, a long period of time is required to obtain single crystal magnetite. In addition, due to using a batch method, in order to obtain magnetite in the tank, it is difficult to make the reaction while maintaining the molar ratio Fe2+/Fe3+ of Fe2+ ions and Fe3+ ions strictly at Fe2+/Fe3+=0.5. For this reason, it is difficult to obtain Fe3O4 nanoparticles uniformly, and in some cases, there is a risk that hematite (α-Fe2O3) or goethite (α-FeOOH) is contaminated. Also, because of the batch method, it is very difficult to obtain nanoparticles having uniform particle size distribution and particle shape due to temperature gradient and concentration gradient in the reaction tank for mass production.
On the other hand, Patent Literature 3 discloses a method of collecting single crystal magnetite from an aquatic bacterium having a plurality of single crystal magnetite (Fe3O4) in one body which is called a magnetotactic bacterium. In the method of collecting substances from such organisms, it is difficult to supply single crystal magnetite stably, and such method is considered to be difficult to use industrially.
As methods of producing cerium oxide, a method of co-precipitating cerium (Ce3+) ions or cerium (Ce4+) ions in an alkaline solution, hydrothermal synthesis at a temperature of 200° C. or higher for a long time, and the like have been known. However, there are problems that the obtained cerium oxide particles tend to become coarse, and it is difficult to obtain nanometer order magnetite having a primary particle size of 50 nm or less, and dry heat treatment or high temperature are required.
As characteristics of nanoparticles, particles generally tend to aggregate due to the influence of surface energy as the particle diameter decreases. For improving the dispersibility, high crystallinity is desirable, and particularly, single crystal oxide particles having the most stable surface state in their average particle size are most desirable. By the improvement of crystallinity or single crystals, improvement of durability against temperature, light or solvent and impact resistance and the like are expected, and further characteristics of the particles tend to be uniformized. Thus, methods of producing single crystal cerium oxide particles have been proposed for a long time as follows.
Patent Literature 5 discloses a method of mixing an alkali base with an aqueous solution of cerium (III) nitrate and, and after ripening, subjecting it to a dry heat treatment within a range of 650 to 1,000° C.
Patent Literature 6 discloses a method of precipitating a cerium salt in the presence of a mixed solvent of an organic solvent and water, and subjecting the obtained cerium hydroxide to hydrothermal reaction at 180° C. to 300° C.
In the method as described in Patent Literature 5 or Patent Literature 6, a high temperature and a long period of time are required to obtain single crystal cerium oxide. In addition, due to using a batch method, it is difficult to secure reaction uniformity. For this reason, it is difficult to control crystallinity of each particle because of formation of coarse particles by dry heat treatment, or ununiformity of temperature or concentration in hydrothermal reaction. Also, because of the batch method, it is very difficult to obtain nanoparticles having uniform particle size distribution and particle shape due to temperature gradient and concentration gradient in the reaction tank for mass production
In order to solve the above problems, the present applicant proposed a method of producing magnetic microparticles by introducing at least two kinds of fluids to be processed in the space between the processing surfaces which are disposed so as to face each other, being capable of approaching to and separating from each other, at least one of which rotates relatively to the other, and mixing the fluids (Patent Literature 4). In the invention of Patent Literature 4, magnetic microparticles such as black iron oxide (Fe3O4: magnetite) and yellow iron oxide (FeOOH: goethite) are obtained by reacting a magnetic law material and a magnetic microparticle precipitation agent in the thin film fluid formed between the above processing surfaces. Because uniformity of temperature is high in the thin film fluid, and uniformity in stirring of the reaction tank is very high, Patent Literature 4 provides a method of producing magnetic microparticles which forms monodispersed magnetic microparticles, and does not have clogging of products due to self-discharging ability, does not require high pressure, and has high productivity.
In order to solve the above problems, the present applicant proposed a method of producing ceramic nanoparticles by introducing a fluid containing a ceramic raw material and a fluid containing a pH adjusting agent in the space between the processing surfaces which are disposed so as to face each other, being capable of approaching to and separating from each other, at least one of which rotates relatively to the other, and mixing the fluids (Patent Literature 7). In the invention of Patent Literature 7, ceramic nanoparticles are obtained by hydrolyzing a ceramic raw material in the thin film fluid formed between the above processing surfaces. Because uniformity of temperature is high in the thin film fluid, and uniformity in stirring of the reaction tank is very high, Patent Literature 7 provides a method of producing ceramic nanoparticles which forms monodispersed ceramic nanoparticles according to the purpose, and does not have clogging of products due to self-discharging ability, does not require high pressure, and has high productivity.
Patent Literature 4 discloses that control of particle diameter or monodispersity, or crystallinity or crystallinity index of the obtained magnetic microparticles can be adjusted by changing rotation speed of the processing surface, distance between the processing surfaces, and flow rate and temperature of the thin film fluid, or the raw material concentration. Patent Literature 7 discloses that control of particle diameter or monodispersity, or crystalline form of the obtained ceramic nanoparticles can be adjusted by changing rotation speed of the processing surface, distance between the processing surfaces, and flow rate of the thin film fluid, or the raw material concentration or temperature and the like.
Therefore, the present inventors have continued the study to improve the crystallinity of the oxide particles, and more desirably to obtain single crystal oxide particles by controlling these conditions. However, under relatively small pressure conditions (0.10 MPaG or less), single crystal oxide particles could not be obtained simply by controlling temperature of the thin film fluid. After that, as a result of intensive study by the present inventors after trial and error, the present inventors have found that crystallinity of the oxide particles can be remarkably improved by the conditions that the temperature of each fluid introduced between the processing surfaces or the temperature at the time of mixing each fluid is set to a temperature higher than a predetermined temperature, and particularly the conditions that under a relatively high pressure condition (higher than 0.10 MPaG), the temperature of each fluid introduced between the processing surfaces or the temperature at the time of mixing each fluid is set to a temperature higher than a predetermined temperature, and thus the present inventors have reached the present invention. For example, the present inventors have found in the production of magnetite particles that crystallinity of the magnetite particles can be remarkably improved by the conditions that the temperature of the magnetite raw material fluid introduced between the processing surfaces is set to a temperature higher than a predetermined temperature, and particularly the conditions that under a relatively high pressure condition (higher than 0.10 MPaG), the temperature of the magnetite raw material fluid introduced between the processing surfaces is set to a temperature higher than a predetermined temperature. In the production of the cerium oxide particles, the present inventors have found that crystallinity of the cerium oxide particles discharged from the space between the processing surfaces can be controlled by changing at least one of the temperature of the cerium oxide precipitation solvent introduced between the processing surfaces and the temperature at the time of mixing the cerium oxide raw material liquid and the cerium oxide precipitation solvent. Specifically, the present inventors have found that crystallinity of the cerium oxide particles discharged from the space between the processing surfaces can be controlled by setting at least one of the temperature of the cerium oxide precipitation solvent introduced between the processing surfaces and the temperature at the time of mixing the cerium oxide raw material liquid and the cerium oxide precipitation solvent higher than a predetermined temperature. Particularly, the present inventors have found that crystallinity of the cerium oxide particles can be remarkably improved by the conditions that the condition that under a relatively high pressure condition (higher than 0.10 MPaG), at least one of the temperature of the cerium oxide precipitation solvent introduced between the processing surfaces and the temperature at the time of mixing the cerium oxide raw material liquid and the cerium oxide precipitation solvent higher than a predetermined temperature.